Particle size distribution measuring apparatuses adapted to measure the particle size distribution of particles contained in a sample include ones of various types as disclosed in, for example, Patent Literatures 1 and 2.
Among such particle size distribution measuring apparatuses, there are ones of a so-called dynamic light scattering type. Further, among the dynamic light scattering type particle size distribution measuring apparatuses, there has been known a continuous measurement type particle size distribution measuring apparatus adapted to successively sample a portion of a sample to introduce the sampled portion into a cell, and measure particle sizes at every time of sampling (although since a certain measurement time is required for one time of particle size distribution measurement, it is strictly not the continuous measurement, but successive measurement).
When performing the measurement, particle sizes in a portion of the sample to be sampled and the concentration of the portion of sample to be sampled gradually change at every time of sampling, and the particle size distribution of the sample may not be identified.
For example, as disclosed in Patent Literature 3, in the case of a slurry type sample containing polish for polishing semiconductor wafers, such a phenomenon may occur. That is, unless sufficiently stirred, slurry causes a phenomenon where a particle size distribution differs locally. As a result, for example, depending on a sampling point in a storage tank of the slurry, the particle size distribution gradually changes with time.
For example, when sampling a portion of the slurry from the upper part of the storage tank in a state where in the slurry stored in the storage tank, a particle group having smaller particle sizes is unevenly distributed in the upper part, and in the bottom part, a particle group having larger particle sizes is unevenly distributed, inevitably, the particle size having smaller particle sizes is first introduced, and then with time, the group having larger particle sizes is gradually introduced. Accordingly, as described above, at every time of sampling, the particle size distribution gradually changes.
In such a case, as described in claim 3 of Patent Literature 1, usually, a particle size distribution is intermittently measured multiple times while continuously introducing a portion of a sample, and by combining particle size distributions obtained at the respective times of measurement, the particle size distribution of the whole of introduced portions of the sample is calculated.
Meanwhile, such a combination-based method for obtaining a particle size distribution has the advantage of being able to more accurately measure the particle size distribution. The reason for this is as follows.
That is, the spectrum of a particle size distribution is eventually obtained by software-based fitting. However, when there are multiple particle size peaks, the fitting is more difficult to perform, and for example, a smaller peak is hidden in an adjacent larger peak, thus causing a limit on the measurement accuracy of the smaller peak. On the other hand, the combined particle size distribution measurement makes it possible to perform easy fitting with high accuracy because the number of particle size peaks is small at every time of measurement.